In a typical gas turbine, the turbine rotor is formed by stacking rotor wheels and spacers, the stacked plurality of wheels and spacers being bolted one to the other. Rabbeted joints are typically provided between the spacers and wheels. In more advanced gas turbines, cooling circuits are provided through the rotor for cooling the buckets. For example, cooling steam may be provided through an aft shaft forming part of the rotor assembly for flow along the rim of the rotor to the buckets of one or more of the turbine stages to cool the buckets. Spent cooling steam also flows from the buckets in a return path along the rim of the rotor and through the aft shaft.